The purpose of the main rotor of a rotorcraft is to provide the rotorcraft with lift, and also with propulsion. Conventionally, such a main rotor, driven by a suitable engine, has a rotor shaft that serves to cause a plurality of blades to rotate by means of a rotor hub.
In order to provide lift and/or propulsion, the pitch of the blades, i.e. the angle of inclination of the blade sections relative to the plane of the rotor hub, is adjusted by means of pitch control rods and swash plates surrounding the rotor shaft, the swash plates comprising in principle a rotary plate associated with the pitch control rods and a non-rotary plate.
The non-rotary plate is situated beneath the rotary plate and imparts movements thereto. Thus, the rotary plate follows all of the movements of the non-rotary plate and transmits them to the blades via the pitch control rod.
Thus, with the help of a plurality of servo-controls, e.g. connected to the structure of the rotorcraft and to the non-rotary plate, swash plates are capable of being moved in translation along the axis of the rotor shaft and also of being tilted relative to said rotor shaft. Usually, such movement in translation and in tilting is achieved with the help of a device including a ball joint capable of sliding along the shaft of the rotor, and having the swash plates arranged thereon.
In order to optimize the dimensioning of the various mechanical parts, the non-rotary plate is secured to the structure of the rotorcraft, in particular with the help of at least one stationary scissors linkage that consequently serves to prevent it from turning.
The same considerations lead to providing the rotary plate with at least one rotary scissors linkage connected to the rotor shaft in order to ensure that the rotary plate is constrained to rotate at the same speed as the rotor hub.
A scissors linkage, whether rotary or non-rotary, generally comprises two hinged arms. More precisely, it comprises a primary arm and a secondary arm, the primary arm being connected, depending on the configuration, either to the rotor shaft or to the structure of the rotorcraft.
The primary arm comprises a central portion and two U-shaped ends, i.e. ends each having a bottom wall and two side branches, the bottom wall of each end being secured to the central portion.
The secondary arm is then disposed between the two side branches at a first end of the primary arm which is secured thereto by means of a pin. The connection to the rotor shaft or to the structure of the rotorcraft is provided in similar manner via the second end of the primary arm.
In normal operation, there is no risk of interference between the primary arm and the secondary arm or between the primary arm and the mount on the rotor shaft or on the structure of the rotorcraft, referred to for convenience below as the “mount” regardless of the configuration, i.e. whether associated with the rotary scissors linkage or the stationary scissors linkage.
However the same does not apply when certain operations are being performed, in particular maintenance operations in which an operator can be caused to move the scissors linkage beyond its normal range of movement. Impacts are then sometimes to be observed between the first and second ends of the primary arm and respectively the secondary arm or the mount, thereby naturally leading to damage to the primary arm, for example. This drawback is particularly troublesome since any damage is situated in a zone that is highly stressed in terms of vibratory fatigue, which leads to a reduction in the lifetime of the damaged mechanical part.
In this context, the manufacturer protects the various elements, the mount and also the primary and secondary arms, by means of anti-impact paint. Such paint is very brittle so as to perform its function of revealing any damage.
Nevertheless, the paint is effective for a single impact only, since multiple impacts have the consequence of causing the paint to flake away, thereby leaving the scissor linkage “bare” and thus vulnerable. As a result it is necessary to disassemble the scissor linkage and repaint it as soon as a single impact has occurred, where such a procedure is manifestly expensive, constraining, and penalizing in terms of rotorcraft non-availability.